1. Field of the Invention
The present invention relates to a zoom lens system for a compact camera, and in particular, relates to achieving a high zoom ratio, and achieving miniaturization of the zoom lens system.
2. Description of the Related Art
In a zoom lens system for a compact camera, there is no need to provide a long back focal distance, unlike a zoom lens system for a single lens reflex (SLR) camera which requires a space for providing a mirror behind the photographing lens system. Accordingly, a compact camera generally employs a telephoto-type lens system in which positive and negative lens groups are provided in this order from the object, while a SLR camera generally employs a retrofocus type lens system in which negative and positive lens groups are provided in this order from the object.
In recent years, there have been increasing demands for further miniaturization and a higher zoom ratio in zoom lens systems for compact cameras. In order to satisfy the demands for miniaturization, it is preferable to employ a two-lens-group zoom lens system having a small number of lens elements. However, if an attempt is made to obtain a zoom ratio of more than 3.5, a three-lens-group zoom lens system has to be used because a two-lens-group zoom lens system has a limit in obtaining well-balanced aberrations at focal length points in a range determined by the short focal length extremity and the long focal length extremity. However, a conventional three-lens-group zoom lens system is not suitable for miniaturization, since the overall length is longer, and the diameter of the front lens group is larger, compared with a two-lens-group zoom lens system.
It is an object of the present invention to provide a three-lens-group zoom lens system of a telephoto type, which attains a zoom ratio (i.e., the focal length at the long focal length extremity/the focal length at the short focal length extremity) of about 3.5 to 4 and whose overall length and diameter of the most object-side lens element are short and small enough to the extent that the same are equivalent to those of a two-lens-group zoom lens system.
In order to achieve the above mentioned object, there is provided a zoom lens system including a negative first lens group, a positive second lens group, and a negative third lens group, in this order form the object, and whereby zooming is performed by moving the first through third lens groups along the optical axis. Further, the zoom lens system satisfies the following condition:
0.05 less than d12W/fw less than 0.35xe2x80x83xe2x80x83(1)
wherein
d12W designates the distance between the first lens group and the second lens group at the short focal length extremity; and
fW designates the focal length of the entire lens system at the short focal length extremity.
The zoom lens system preferably satisfies the following condition:
0 less than log10Z12/log10Z less than 0.15xe2x80x83xe2x80x83(2)
wherein
Z12=f12T/f12W; 
Z=fT/fW; 
f12T designates the resultant focal length of the first lens group and the second lens group at the long focal length extremity;
f12W designates the resultant focal length of the first lens group and the second lens group at the short focal length extremity; and
fT designates the focal length of the entire lens system at the long focal length extremity.
The zoom lens system preferably satisfies the following condition:
4 less than fT/f12T less than 6xe2x80x83xe2x80x83(3)
wherein
fT designates the focal length of the entire lens system at the long focal length extremity; and
f12T designates the resultant focal length of the first lens group and the second lens group at the long focal length extremity.
The zoom lens system preferably satisfies the following condition:
4 less than fT/|f3G| less than 8xe2x80x83xe2x80x83(4)
wherein
f3G designates the focal length of the negative third lens group.
The present disclosure relates to subject matter contained in Japanese patent Application No. Hei-10-368173 (filed on Dec. 24, 1998) which is expressly incorporated herein in its entirety.